Working Stresses [Reprint 2022 ed.] 9781978811874

Table of contents :
INTRODUCTION
WORKING STRESSES FOR STATIC LOADS
WORKING STRESSES FOR FLUCTUATING LOADS
WORKING STRESSES FOR CREEP CONDITIONS
CONCLUSION
FIGURES
REFERENCES

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WORKING STRESSES

Working Stresses JOSEPH MARIN

NEW BRUNSWICK

RUTGERS UNIVERSITY PRESS 1940

RUTGERS UNIVERSITY STUDIES IN ENGINEERING NUMBER ONE

COPYRIGHT 1940 BY T H E TRUSTEES OF RUTGERS COLLEGE IN N E W JERSEY PRINTED IN THE UNITED STATES OF AMERICA

WORKING STRESSES

Working Stresses INTRODUCTION THE selection of the correct working stress value for the design of a structure or machine is one of the major considerations in obtaining a safe and economical construction. There are many factors involved in deciding the value of this stress. The reliability of the material is important since nonuniformity of the material, presence of flaws, and other uncertain factors modify greatly the strength of a material. The exactness with which the loads are known and whether such loads are steady or variable, influences the working stress value. Another important consideration is the accuracy of the theory used for computing the stress in the machine or structural part to be designed. Simplifying assumptions may produce errors in the stress calculations. There are also certain consequences of time which modify this stress value, such as, corrosion, deterioration and creep. Furthermore, in selecting the working stress, the seriousness of failure is an important factor. Finally, the effect of a combined state of stress on the failure of a material will in some cases influence greatly the working stress. This volume deals mainly with a discussion of this last factor. The failure of materials subjected to static, fatigue and creep loadings in the case of combined stresses will be discussed. With this as a basis, the working stresses under such loadings can be considered. WORKING STRESSES FOR STATIC LOADS 1. Failure of Materials

Subjected to Simple

Tension

A member subjected to simple static tensions fails to be of service when the stress or deformation reaches some limiting value. In defining this point of failure two types of materials are to be distinguished, namely, ductile and brittle materials. For a ductile material, such as steel, the stress-strain graph may be of two types—one in which there is a lower and upper 3

WORKING STRESSES

4

yield point and another in which there is no definite yield point. For the first type, the lower yield point defines best the failure of the material since it is more stable in value than the upper yield point (1).* For the second type of ductile material failure is best defined by a proof stress—a stress producing a specified limiting unit strain. The lower yield point or proof stress defining failure will be designated by a. In the case of a brittle material such as cast iron, since there is no definite region of yielding, it is necessary to define failure by the ultimate stress. 2. Failure

of Materials

Subjected

to Static

Combined

Stresses

There are many machine and structural units subjected to loadings such that there are produced stresses in more than one direction. The magnitude of these stresses can usually be determined at a point in the member and represented in turn by the equivalent principal stresses—normal stresses acting on planes which are free from shearing stresses. For simplicity, in this discussion the two-dimensional case of stress, as shown in Figure 1, will only be considered. The values of the principal stresses